1. Technical Field
The purpose of this invention is a magnetic field sensor with magnetoresistance. Its applications are mainly for the measurement of weak fields, varying from about one nanotesla to a few milliteslas, and for magnetic recording particularly for reading.
2. State of Prior Art
A magnetic field sensor with magnetoresistance is a device that schematically comprises a magnetic circuit with a main air gap capable of detecting the field to be measured, and a secondary air gap (or read gap) associated with a magnetoresistance. (In some cases, the main air gap and the secondary air gap are coincident). The detected magnetic field is directed by the magnetic circuit and is coupled to the magnetoresistance in the secondary air gap. The value of the resistance of the magnetoresistance will depend on the field that passes through it, such that information about the detected magnetic field can be obtained by measuring this resistance.
This type of sensor can be made in different forms, and in particular comprises magnetic heads with thin layers, suitable for reading or writing. In a magnetic head with thin layers, the magnetic circuit is composed of one or several thin layers (typically of the order of a micron) made of a material with high permeability. A magnetic head with thin layers may have a horizontal structure when the thin layers plane is parallel to the recording medium (disk or tape) during operation, or a perpendicular structure in which case this plane is perpendicular to the recording medium. An inductive type of writing operation can be designed for each of these two types of head, by passing a current through a conducting coil surrounding the magnetic circuit.
The magnetoresistance that is used for reading will be denoted MR in the following, using a standard abbreviation in this technique.
FIG. 1 attached shows an example of a horizontal type of magnetic head with thin layers as described in document FR-A-2 712 420. This figure shows a substrate 10, a rear magnetic layer 12, two magnetic pillars 16-1, 16-2, two intermediate magnetic layers 20-1, 20-2 defining a secondary air gap 22, these layers 20-1, 20-2 becoming narrower towards the air gap 22 than towards the pillars 16-1, 16-2, and therefore forming field concentrators. One magnetoresistance 24 is located across the secondary air gap 22, a polarization conductor 26 being placed above it. Two polar parts 28-1, 28-2 define a main air gap 30. A conducting coil 18 surrounds the pillars 16-1, 16-2. This head is capable of relative movement with respect to a magnetic recording support 40.
Document FR-A-2 712 420 contains references to other types of horizontal magnetic heads with thin layers, in which the MR is placed close to the main air gap or close to an air gap formed in the rear magnetic layer.
Furthermore, the article by K. YAMADA entitled xe2x80x9cMagnetoresistive Head for High Density Magnetic Recordingxe2x80x9d published in the xe2x80x9cIEEE Translation Journal on Magnetics in Japanxe2x80x9d review, vol. 8, No. Apr. 4, 1993, pp. 260-268 describes a vertical type of magnetic head which is illustrated in the attached FIG. 2. This shows a head comprising a magnetic circuit with two polar parts 50-1, 50-2 separated by a main air gap 52, two side arms 54-1, 54-2, and a rear closing part 56. Two secondary air gaps are formed in the side arms in which two MR 60-1, 60-2 are located.
The recording support 70 is placed facing the head, which is perpendicular to the said support.
There are simpler sensors composed of an MR placed between two magnetic screens, adjacent to these sensors in the form of a magnetic write/read head. The magnetic circuit is therefore reduced to these two screens.
The article by HIROTSUGU FUKUOKA et al. entitled xe2x80x9cEffect of Magnetic Saturation on Reproducing Characteristics of Magnetoresistive Headsxe2x80x9d published in xe2x80x9cIEEE Transactions on Magneticsxe2x80x9d, vol. 30, No. Jul. 4, 1994, pp. 1345-1349, thus describes a sensor that is shown in the attached FIG. 3. This sensor comprises two magnetic screens 70-1, 70-2 and a magnetoresistance 72. A conductor 75 is placed flat on this magnetoresistance. The assembly moves in front of a recording support 80. In a way, the main air gap that detects the field from the support, is coincident with the secondary air gap containing the MR.
The article by R. YAMADA, mentioned above, also describes a similar device.
Magnetic sensors in which the MR is within the main air gap involve risks of the MR becoming saturated, which are even more severe with magnetic heads that operate in write and in read.
Furthermore, all these sensors according to prior art have some disadvantages. In magnetic heads like those described in FIG. 1, there are magnetic field leakages that weaken the measurement signal. These leakages also exist in devices like those shown in FIG. 2, and furthermore there is not much space in which to house the MR. With the sensors in FIG. 3, the MR is located too close to the recording support, resulting in accelerated wear, in addition to potential electrical breakdown problems. Furthermore, the size of the MR is limited by the size of the air gap, which itself depends on the density of information recorded on the support.
The purpose of this invention is to correct these disadvantages.
According to the invention, the magnetic circuit is made wider at the secondary air gap at the location of the MR, in order to minimize field losses (the active flux, in other words the flux crossing the MR, is maximum) while maximizing the available space to position the MR (which increases the signal and the signal-to-noise ratio). Saturation risks are thus also reduced due to the fact that the field is reduced due to the increased width at the secondary air gap. In the past, those who are skilled in the art would attempt to concentrate the magnetic field in the MR and therefore reduce the section of the magnetic circuit acting as a guide close to the read air gap in order to increase the flux at the sensor. This can be clearly seen in FIG. 1, described above, in which layers 20-1, 20-2 act as concentrators. Consequently, it also increased the reluctance of the magnetic circuit and the risks of the MR becoming saturated. In other words, an expert in the subject would attempt to increase the field level to increase the signal. The original concept for the invention is to increase the integration section while taking advantage of the fact that MRs include the signal in their volume. Therefore in the invention, the width of the magnetic circuit is increased at the read air gap.
More precisely, the purpose of this invention is a magnetic field sensor comprising a magnetic circuit with a main air gap through which the field to be detected can pass, this magnetic circuit being interrupted by at least one secondary air gap provided with a magnetoresistance, this sensor being characterized in that the magnetic circuit has a hollowed out shape on each side of the secondary air gap.
The sensor according to the invention may be in several shapes, for example either in the general form of the horizontal type of magnetic heads with thin layers, or in the general form of the vertical type of magnetic heads with thin layers, or in the form of magnetic screen sensors.